Many machines, such as turbines, have a centrally disposed rotating portion that rotates within a stationary portion, casing or shell. In turbines for example, a typical problem known as “rubbing” arises when a rotating portion of the turbine, such as a rotor, comes into contact with another component of the turbine, such as the stationary casing, when the components are not in contact with each other under normal circumstances. Rubbing often gives rise to unwanted and potentially dangerous conditions in rotating machinery. For example, one of the harmful effects of rubbing can be witnessed in steam turbines. Typically, steam turbines include a rotor with rows of turbine blades that rotate between rows of stator blades. The tips of the turbine blades are adjacent to an inside surface of the casing for the turbine. During normal operation, the tips of the turbine blades do not rub against the casing. However, a slight deformity in the turbine casing, rotor shaft, inner casing or any other component can cause the turbine blades to rub against the stationary casing of the turbine. Rubbing between the turbine blades and the stationary component in a steam turbine can damage the turbine components, while increasing the clearance between the turbine blades and the stationary casing to prevent rubbing can reduce the efficiency of the flow through the steam turbine.
Rubbing may occur due to an unbalance in the rotor, thus giving rise to abnormal vibrations in the turbine. In conventional turbines, a plurality of vibration sensors may be mounted at various points on the turbine system to monitor signature vibrations indicative of rubbing. Other techniques for monitoring rubbing may include the use of particular sensors for detecting the occurrence of acoustic emissions within the metal parts of the turbine, such acoustic emissions being generated as a result of certain abnormal operating conditions in the turbine. Such conventional techniques, however, have often proved to be inaccurate as it is extremely difficult to distinguish between the vibrations caused by rubbing and the vibrations caused by the turbine itself. More specifically, the vibrations detected by the sensors usually include the vibrations resulting from the various components of the turbine itself.
Accordingly, there is a need for methods, systems and apparatus for detection of rubbing in machines.